Automatic pilot for helicopters



Aug. 23, w49; w. A. AYRES ETAL AUTOMATIC -PILOT FOR HELICOPTERS Filed Nov. 1o, 1943 Patented Aug. 23, 1949 2,479,549 AUTOMATIC PILOT FOR HELICOPTERS Waldemar A. Ayres, Kew Gardens Hills, andi Mortimer F. Bates, Brooklyn, The Sperry Corporation,

Walle Application November 10, 1943, Serial No.

N. Y., aissgnors-to4 a-corporation ofDela- This invention relates generally to an automatic pilotl for aircraft and particularly concerns automatic control` of an aircraft having a sustaining propeller or rotating wings. The invention. is particularly useful for automatic control of helicopters and is described herein as applied to a helicopter of the single rotor type having an off-set torque compensating propeller.

According to the present invention the lift characteristic of the sustaining propeller is controlled to correct for the displacement of the aircraft' from a predetermined attitude This displacement may be occasioned by aerodynamic instability of the aircraft or may be introduced intentionally by changingthe reference attitude. In addition to stabilizingthe aircraft in a selected attitude, the thrust axis of the sustaining propeller may'be adjusted to control the horizontal component of thrust thereby controlling horizontal movement of the aircraft.

Horizontal movement may be combined with vertical movement by changing the collective pitch of the rotating wings that sustain theV craft in the air. Adjustment of the lift is controlled by an altitude sensitive device which maybe set for a selected altitude. It then controls mechanism for changing a collective pitch of the sustaining wings to move the aircraft toward the selected altitude and then controls the lift in a manner to maintain the aircraft atA that altitude.

These controls for changing the lift and the lift characteristic of the sustaining propeller may also change its torque. In order to prevent this change in torque from causing rotation of the aircraft about the axis of the sustaining propeller, it is necessary to change ther pitch of the torque compensating propeller which determines the heading of the aircraft. A directional instrument such as a gyroscope controls the thrust of the torque compensating propellerto maintain a predetermined heading of the aircraft. If a change in the torque of the sustaining propeller causes the aircraft to begin to turn, the torque compensating propeller will beY actuated by the directional instrument in a manner to return the aircraft to its original heading and maintain it at such heading. The directional instrument may be adi-usted manually to maintain the heading of the craft in any desired direction.

The direction of horizontal motion of the aircraft is determined by two factors, namely, the heading of the aircraft and the position of the thrust axis of the sustaining propeller relative to the aircraft. As is well known, va helicopter may be moved horizontally in any direction, that is, forward, backward or laterally. The direction of motion is determined not only bythe headingof the craft but also by the vhorizontal movement of the aircraft relative to its heading. If it is desired to proceed along a certain course, the heading may be set along that course or at a specific angle relative thereto. adjustment of the attitude control device may cause the thrust axis of the sustaining propeller to have a forward horizontal component or to havel the horizontal component at a specific angle relative to the fore and aft axis of' the aircraft. In both cases, the direction of motion is-determined jointly by the attitude device and the directional instrument.

A major object of the invention is toprovide an automatic pilot for an aircraft having a sustaining propeller, for maintaining the aircraft at a predetermined altitude, in a predetermined attitude and on a predetermined course.

Another object of the invention is to provide an automatic pilot for an aircraft'having a sustaining propeller in which the aircraft is maintained at a predetermined attitude by automatic control of the lift characteristic of the sustaining propeller.

A further object of the invention is to provide an automatic pilot for ,a helicopter in which horizontal motion is controlledby an attitude sensitive device that adjusts theY horizontal component of the thrust axis of a sustaining propeller.

A further object of the invention is to provide an automatic pilot for a helicopter in which the direction of motion is controlled jointly by heading and attitude sensitive devices.

A further object of the invention is to provide an automatic pilot for an aircraft having a sustaining propeller in which a directional instrument is used to control the action of a torque compensator for controlling the heading of the aircraft.

A further object of the invention is to provide an automatic pilot for an aircraft having a sustaining propeller in which an altitude sensitive device controls the collective vpitch of the sustaining propeller in a .manner to automatically move the aircraft toward or maintain it at a predetermined altitude.

A further object -of the invention is to provide an automatic pilot for a helicopter in which the lift of a sustaining propeller is controlled by an altitude sensitive device to maintain the aircraft at apredetermined altitude and changes in torque of the sustaining propeller are automatically compensated by a torque compensator under the control of a directional instrument.

A still further object of the invention is to provide automatic pilot for a helicopter in which horizontal and vertical components of motion are controlled by changes in the lift characteristic and the lift of a sustaining propeller by attitude and altitude sensitive devices, respectively.

Other objects and advantages of the invention will become apparent from the following specication, taken in connection with the accompanying drawings, whereinV Fig. 1 is a schematic diagram of an-automatic pilot embodying the invention asv applied to a single rotor helicopter having a torque compensating propeller, some .of shown in section and others 1n perspective, for purposes of clearness;

Fig. 2 is a partial top plan view of the control apparatus for the torque compensating propeller shown in Fig. 1;

Fig. 3 is a wiring diagram illustrating one circuit arrangement for the follow-up control circuits of Fig. 1; and

Fig-4 is a wiring diagram illustrating another circuit arrangement for the follow-up control circuits of Fig. 1. The automatic at that altitude.

Y A'directional control knob I5 may be set for a selected heading ofthe aircraft which preferably corresponds to an angular displacement between the longitudinal axis of the aircraft and the directional axis of the gyroscope. The attitude control I 3 includes two adjusting knobs I1 and I8 for setting components of a selected attitude into angular components about the longitudinal and athwartship by a portion of the fuselage 25 of the aircraft. In the case of a helicopter, the sustaining propeller is power driven as by a gear 26 on the shaft 23 which meshes with a pinion 21 carried by a shaft 28 from a gear box or transmission 219. A suitable power plant 3 I, such as an internal combustion engine, drives through shaft 32 to the transmission 29 for driving the sustaining propeller 2l.

In order to produce vertical movement of the aircraft. the collective pitch of the wings 22 is increased or decreased to-cause the aircraft to ascend or descend. The lift of the sustaining propeller is automatically controlled by the position of aneroid bellows Il which depends upon the altitude of the aircraft and the position of altitude control knob I4 that may be set manually for a selected altitude. The bellows I I operates through a rack 33 and pinion 34 to rotate shaft 35 which positions the rotor of a suitable transmitter or rotary transformer 36 that may be of any suitable type, such as a Selsyn or Telegon This rotor winding may be connected in a conventional manner to a source of alternating cur- Y the direction and rent potential 38. The stator winding of the signal generator 36 may be connected as by leads 39, 39 to stator winding of a corresponding receiver or rotary transformer 4I which is also of a conventional type.

In the case of a Selsyn or Telegon, a voltage is induced in the rotor Winding of the receiver 4i, displaced in phase according to the angular displacement of the rotor windings in the signal transmitter 36 and the receiver 4I relative to their respective stator windings. The rotor-winding of the receiver 4I may be connected as by leads 42 to a control amplifier 43 which is also of conventional design, including a phase detector for producing a voltage corresponding to amount of phase displacement between the voltage induced in the rotor Winding of the receiver 4I and the voltage of the source 38, the latter being connected to the control amplier as by leads 44. The voltage thus produced operates through an amplifier circuit to control the currents in the field windings of a motor 45 which are connected by leads 46 to the control ampliner 43. In this manner, the motor 45 turns in a direction and at a rate corresponding to the direction and amount of the phase displacement of two voltages which corresponds to the displacement of the rotors in the transmitter 36 and the receiver 4I from a synchronous position.

The motor 45 rotates a shaft 41 through gearing 48 to rotate a nut 49 supported in a bracket 5I that is fixed to the body of the aircraft. The nut 49 engages a threaded rod 52 which drives arranged to move a shaft 53 up or down, de

pending upon the direction of rotation of the nut 49. The shaft 53 rotates with the propeller 2| and has a cap portion 54 on which a lever 55 is pivoted as at 56. One end of the lever 55 is pivotally connected to a link 51 which is connected to a crank 58 mounted on the support for one of the blades 22.

The blades or rotating wings 22, 22 are pivotally mounted as at 59 for rotation about a horizontal axis extending radially from the supporting shaft 23. The opposite end of the lever 55 is attached to a link 6I which is connected to a wobble plate 62 that is adjustably positioned to cyclically vary the pitch of the blades 22, 22 as they are rotated by the shaft 23 to change the lift characteristic of the sustaining propeller, as will subsequently be explained in greater detail.

From the foregoing description, it will be apparent that movement of the shaft 53 either upwardly or downwardly causes crank 58 to change the pitch of the propeller blades 22. Although not shown herein, it is the usual practice to change Ythe engine throttle simultaneously with the pitch of the propeller blades so as to maintain the rotor speed substantially constant when the pitch of the propeller is increased, and vice versa. Although the details are not shown in the drawings, any desired number of blades may be connected in a similar manner to simultaneously change the pitch of all of them by movement of the shaft 53.

As previously stated, the transmitter 36 and the receiver 4I may be of any suitable type to form a Vtransmission system for actuating amplifier 43 to control the motor 45. One arrangement of a follow-up circuit for this purpose is shown diamitter 36 and stator winding 12 is connected to stator winding 13 of the receiver 4I. The voltof the potentiometer slider 30| by the shaft '35. Thusthe follow-up ratio of the system may be varied in any desired manner by adjustment of thev potentiometer slider 322 to vary the voltage applied across the terminals of the potentiometer 3| 8.

Although two specific control circuits have been illustrated and described, it is contemplated that any desired type of control circuits may be used to operate the pitch mechanism from the altitude sensitiveV device Without departing from the invention.

The mechanism for changing the pitch of the sustaining propelleris, of course, providedrwith suitable limit stops for the blades so maximum and minimum angles of attack are not exceeded. The altitude control apparatus is so arranged that for changes in altitude exceeding a definite amount, for example, 100 or 200 feet, the pitch of the sustaining propeller will be adjusted to its maximum or minimum angle of attack. lFor smaller changes in altitude, the angle of attack depends upon the change desired so the vertical velocity will be commensurate with the distance to be traveled.

In order to prevent the torque of the shaft 23, which drives the sustaining propeller 2|, from rotating the aircraft about the axis of the shaft 23, a torque compensating propeller 951 is also driven from transmission 29 as by a shaft 99 whichV drives through gearing A housing |02 extending from the aircraft supports the torque compensating propeller in an offset position relative to the axis of the sustaining propeller 2|. The torque compensating propeller 98 has a horizontal thrust which, assuming a 'constant angular velocity, depends upon the pitch of the propeller 98.

As the pitch of the sustaining propeller is adjusted to change or maintain the aircrafts altitude, the torque of the shaft 23 may also be changed. Unless the pitch of the torque compensating propeller 98 is then adjusted, the aircraft will begin t0 turn due to the torque differential exerted upon the aircraft by the shaft 23 and the compensating propeller 98. Y

In order to avoid this diculty and also to maintain a constant heading for the aircraft regardless of other influencing forces, a directional instrument such as the directional gyro I2 is employed to automatically control the pitch of the torque compensating propeller 98 sov the aircraft Will at all times maintain a selected heading.

Adirectional instrument such as a directional gyro |2 of a conventional type including appropriate flux gates, or a meridian seeking gyro is supported for movement about a vertical axis as by shafts |04 and |05. A conventional follow-up such as a base |06 is actuated by a` Worm lill' which meshes with a worm gear |08 to maintain the position of the base aligned with that of the gyro regardless of movements of the aircraft. This is accomplished by a conventional pick-off which includes an E transformer vthat has its center leg energized by a winding connected to a suitable source of alternating current. Windings around the other two legs of the E- shapedcore of the transformer |09 are connected in series.

The voltages induced in these tWo series windings are controlled by the position of an armature ||2 which is mounted by an arm I3 extending from the shaft |05. Movement of this armature ||2 relative to the core of the trans-y former induces a differential Voltage in the windings on the outside legs which is reversible in phase andvariable in1magnitude, depending on the direction and amount of displacementilof armature ||2 relative tothe core of the -transformer. This voltage is supplied as by leads ||4 to a conventional control amplier H5 which may be similar to the control amplifier 43 shown in detail in Figure 3.

The amplier ||5 controls the current in the field windings of a motor H6 which rotates the Worm |01. `It will be apparent that displacement of the lbaseV 06 relative to the reference position of the gyro causes a diiferential voltage from the E transformer to operate through amplifier ||5 to actuate motor I6 in a. manner which moves the transformer |09 to reduce the said displacement. This follow-up apparatus thereby constantly maintains the base |06 iril a position substantially identical Vwith that of the gyro and thus defines a reference direction. A gear 2| is also driven by a motor ||6 to rotate a gear |22 which drives through shaft |23 to position the rotor Winding of a transmitter |24 which may be similar to the transmitter 35 previously described in connection with the altitude control. The gears |2| and |22 are driven synchronously with the base |06 so their positions as Well as the position of the rotor winding of the transmitter |24 Lat all times correspond to the position of the directional gyro. This rotor winding may be energized from a suitable source |25 of alternating current and the stator Winding of the transmitter |24 may be connected as by lead |22 to one of a pair Vof three phase windings in a rotary transformer |28.

The rotary transformer |28 may be of any suitable design having a pair of three phasewindings, one of which is rotatable relative to the other as by a shaft |29 carrying the course setting knob l5. The remaining winding of the rotary transformer |28 is connected to the stator winding of a receiver |30. The receiver |39 has its rotor winding connected to control amplifier The motor which rotates a nut |35, also mounted on the housing |02. The nut |35 engages thethreaded portion |'38 of a rod |39 that is connectedby a link |4| to one arm of a bell crank lever |42 the otherA arm of which is attached by a suitable coupling |45 to a rod |44 extending through the driving shaft |00 for the propeller 98. The bell crank |42 is pivotally mounted at |40 on the housing |02. Thecoupling |45 permits rotation of the rod |44 with the propeller 98, but is secured |'42 about its of the rod |44.

The blades of the torque compensating propeller 98 are pivotally mounted on driving shaft |00 for adjustment of the arm |41 on the end of the rod |44 is connected 'by a link |48 to a crank |49 on the propeller 98 so longitudinal movement of the rod |44 changes the pitch of the propeller 90.V

The heading control circuitand mechanism described adjusts the pitch of the torque compensating propeller to maintain the craft at a desired headingas determined by the position of andere control knob `|5. This knob positions the rotor Vwinding `of the rotary transformer |28 which a selected course. Obviously a similar result may y be obtained by adjusting the casing of the transmitter |24 to introduce any desired angular r'displacement between the selected course and the position of the directional gyro |2.

The motor |32 is driven according to positional disagreement between thel aircraft and the de'- sired heading as determined by direction control knob l5. In this manner it operates to adjust the pitch of torque compensating propeller 88 in a manner which turns the aircraft toward a Vde'- sired heading. Appropriate limit stops 'are provided for maximum and minimum angles of attack for the propeller. If there is a large displacement of the aircraft from a selected heading it causes the propeller 98 to change to' its maxi'- mum or minimum pitch. As the aircraft approaches the desired heading, or when it is dis-` placed only a small amount from the desired heading, the motor drives in the opposite direc'- tion to slowly return the propeller 98 towardy its normal pitch angle.

The gyro vertical I3` performs a dual function in controlling the attitude of the aircraft. It acts as an attitude sensitive device to change the lift characteristic of the sustaining propeller to maintain a predetermined attitude ofthe aircraft. It also may be adjusted to cause the aircraft to assume a selected attitude thereby changing the position of the thrust axis of the sustaining propeller to introduce a horizontal thrust component and cause horizontal motion of the aircraft. The direction and amount of this horizontal motion is determined by the attitude at which the gyro vertical controls the aircraft'.

The gyro vertical |3 may be of conventional de` sign which includes a rotor mounted' in a vgi'mbal |52 for movement about tvvo axes. A shaft |53 projects from the gyro rotor |'5|, moving a bail |54 according to relative movement' of the aircraft relative tothe gyro about one axis; Movement of the bail I`54fturns a rotor winding of a pickoff or transmitter |55`vvhich is of conventional design and may be similar to the transmitter 36 previously described; Movement of the aircraft about a second axis causestlfie gimibal |52 to turn a rotor winding of a second pickoi or transmitter |56 that is also of a conven= tional type such as the transmitter 36 previously described. 5

The housings of the transmitters |55 and |55 are adjustable by turning attitude controllg'nbs |1y and |8 which operate through worms |58 and |'59` that engage worin gears |6| and |62Von the transmitters |55 and |56, respectively. lThe po"L sition of the stator windings carried by' the house' ings of these transmitters determines the attie tude of the aircraft. j Y j The rotor winding oi the transmitter |55 may be connected to a suitable source |64'of alternat-y irig current and the stator winding of that transmitter may be connected as by leads |65 to 'a' cor; responding stator winding of a receiver |86"that may be similar to the receiver 4|. The rotor winding of the receiver |66 is connected to accin-y ti" 'mpli-er |81 which may besiinilarvtothe amplifier 43 described and' Shown iii detail `acteristic of the. sustaining propeller.

Fig. A3.v The famplier |61 controlsa motor |88 according to the positionsbf therotors of transmitter |55. and receiver |66 relative to their refspective stator windings'. The motor |68 operates through gearing |69 to rotate a nut '|1|, carried in a bracket |12 'fixed to the body of the aircraft. Nut |1| enga-ges a threaded portion |13 of a rod |14 whichis'. pivotally connected to a guide |15 for the wobbleplate 62 that is preferably mounted. as by a .ball joint; for universal movement on th'e shaft 23;

Movement of the rod |14 changes the lift charto change The motor |88 also drives .through gearing |16 vandv |11 to rotate shaft |18 which positions the rotor winding 'of the receiver |86 according to the movement of the rod |14". The displacement of this rod there; fore depends upon the displacement of the aircraft "f-rom the .reference attitude which is predetermined by adjustment of control knob |1 which posit-ions the stator winding of transmitter |55.

In a similar. manner,- the rotor winding of the transmitter |56 may be 'connected to a suitable source |8|` of alternating current potential and its stator connected as` by leads |82 to the stator windingl of a receiver |83. The receiver |83has its rotor winding connected' to control amplifier |84 in a manner similar to that shown in Fig. 3, for the amplifier 43. The; amplifier |84 controls the current in opposite halves of the field winding'. of a motor |85 that drives through gearing |88 to rotate a nut |81 carried by bracket |88 which'lis' flied to the body f the aircraft.` The nut |81 engages' a threaded portion |88 of a rod |9| that pivotally connected' at |82l to the guide |15 for adjustingVV the lift characteristic of the' sustainingv propeller by positioning wobble plate tz. Y

The motor |r also4 drives through gearing |94 and |95 to rotate shaft |86 which positions the rotor winding of the receiver |83. Since centrol of the motor |85V depends upon the relative ponti-@nef transmitter and receiver. it operates as a' follow-iup to position the rod' |`9| according to the' displacement of the aircraft from the predetermined re'ference attitude; u

Control of theV lift characteristic of the pro-v peller is" effected by the" position ofthe wobble plate to 'change the attitude of the aircraft in the following' nrianriei". The rods |14 andv |8| are pivoted to thezgui'de' |15 at points spaced by approximately whereby movement of these rods causes movement f' thewobble plate 62 about tvvil` substantially perpendicular that may correspond tolateraland longitudinai aires' of the aircraft. Y

In anrair'craft having a single sustaining propeller, the' lift characteristic isfusu'ally controlled by cylically vi'rary'ing' the" lift of' the propeller blades individually dering' caen revolution of the propeller.V For' exa, `le, `if the rod |9| is raised; and the me' |14 r drainin aident-ral positcn, the nel; sl' is friovd dpaceown as the wobble plateA 52 rotates the 'de |1,5. This movement of the 5| i lgrlefvratesjtliivilgh the lever 55 yto ascii-iat'e die blade' 2121 about y its Anariz,octal radial axis du geaclir ution of the pOi the attitude of the aircraft. Y.

pellen 'thereby causin angle of;` attack, that is eali revolut ti'ief :a f

@rie han of of attackd' ing tion. 'rits a" s vertical thijls't l1 rection ofthe axis being dependent upon the position of the rwobble plate 62. As the propeller begins to rotate about the horizontal axis, it rotates the entire aircraft thereby effecting a change in its attitude.

Since the position of the wobble plate is controlled by follow-up circuits, according to the displacement of the aircraft relative to a predetermined or reference attitude, as determined by the gyro vertical I3, it changes the lift characteristic of the sustaining propeller in a manner to return the aircraft, by rotating it about a horizontal axis, to said predetermined attitude.

Under some circumstances it may be desirable to change the response ratio of the change in the pitch characteristic to the displacement of the craft from the reference attitude. To accomplish this, potentiometers may be used for pickoffs on the gyro vertical instead of the synchrotransmitters IE and I56. By arranging the potentiometers in a circuit such as that shown in Fig. 4, the response ratio of the control circuits is adjustable by changing the Voltage applied across one of the potentiometers.

As in the case of the other controls, suitable limit stops must be provided for maximum and minimum pitch angles of the propeller blades. For large changes in attitude, the pitch angle of the blades is changed a maximum amount to introduce a maximum permissible change in the lift characteristic of the sustaining propeller. As the displacement between the aircraft and its reference attitude `becomes smaller, the motors |68 and I 85 are actuated by the follow-up circuits described to reduce the change in the lift characteristic. When the attitude of the aircraft corresponds to the reference attitude, the motors will have returned the wobble plate to its neutral position. By this arrangement, the aircraft is stabilized in a predetermined attitude of flight.

In a helicopter, horizontal motion of the aircraft is controlled primarily by the position of the thrust axisof the sustaining propeller. To cause horizontal motion of the aircraft, a horizontal component of the thrust axis is introduced in a direction corresponding to the motion desired. The speed of this horizontal motion depends upon the amplitude of the horizontal component of thrust. When the motion of the aircraft includes a horizontal component, the attitude of the craft changes'so it tilts in the direction of motion.

Since the attitude of the aircraft has a deflnite relation to by appropriate setting of attitude control knobs I1 and I8. Thus, the gyro vertical I3 serves to stabilize the aircraft at a specic The stabilization is eiected by changing the lift characteristic of the sustaining propeller so the aircraft remains in a predetermined atti- The horizontal motion is controlled. by

ing the attitude control knobsV I 'I'and'l8, the vertical component of thrust, is also changed. It is necessary, therefore, to compensate this change in vertical thrust in order to prevent the aircraft from changing altitude when horizontal motion is effected.

If the aircraft deviates from the altitude set into the automatic pilot by control knob I4, a signal Will be produced by the aneroid bellowsl I,

peller blades, thereby changing the lift of the sustaining propeller. This action is eiective to at a substantially constant altitude, since it operates upon a small deviation from the desired altitude.

In addition to changing the vertical propeller is properly ad- If the aircraft rotates due to such a change in torque, it begins to deviate from the desired heading set by direction control knob I5.

If for some reason the torque applied to the sustaining propeller fails, the

may move in any heading. Therefore, the direction of motion is dependent upon the well as upon the heading of the aircraft. For this reason the directional gyro and the gyro vertical operate the direction of motion of the control knob I5 the aircraft.

The automatic pilot described herein operates to completely automatically control ight of an aircraft having a sustaining propeller. The conadjusted to the desired course of peller.

Although the automatic pilot described herein is electrically operated it will be apparent that equivalent hydraulic or mechanical controls may be substituted for any or all of the electrical controls. The particular mechanism for changing the lift, and lift characteristic of the sustaining propeller may also be modified without departing from the invention. If desired, the automatic pilot may be rendered inoperative by opening power circuits to the motors and control circuits, and the aircraft operated by manual controls.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

In an automatic pilot for an aircraft having a sustaining propeller, apparatus for controlling the attitude of said aircraft comprising a gyro for dening a reference attitude, and control mechanism actuated by said gyro responsive to displacement oi said aircraft relative to said reference attitude for changing the lift characteristic of said propeller according to said displacement, said control mechanism including adjustable means for changing the response ratio of said propeller to said displacement.

WALDEMAR A. AYRES. MORTIMER F. BATES.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date Sperry, Jr., et al Feb. 7, 1933 Carlson July 11, 1933 Avery Mar. 5, 1935 Fischel Apr. 9, 1935 Smith Dec. 3, 1935 Carlson June 30, 1936 Koster Apr. 5, 1938 Carlson Jan. 10, 1939 Thiry Feb. 13, 1940 Fischel Feb. 20, 1940 Pitcairn May 30, 1944 

